1. Field of the Invention
The present invention relates generally to a mobile communication terminal having two or more processing modules to support two or more modes, and in particular, to a handover control device and method for, when a traffic handover occurs, reducing time delay resulting from the traffic handover and enhancing a handover success rate.
2. Description of the Related Art
A mobile communication network is being evolved from a code division multiple access (CDMA) network for a second-generation mobile communication service to a wideband code division multiple access (WCDMA) network for a third-generation mobile communication service. The coverage area of the WCDMA network is also expanding; in general, a WCDMA service area is less than a CDMA service area. Therefore, a mobile communication terminal supporting a WCDMA service is being manufactured as a multi mode terminal that can support even a conventional second-generation CDMA service, and has a function of idle handover capable of mutually switching two WCDMA and CDMA modes.
The idle handover function is a function switching operation occurring in a standby state. If a user communicating in the WCDMA mode at a service area providing all WCDMA and CDMA services enters a shadow area providing only the CDMA service without the WCDMA service, communications are cut off, and then, the standby state is switched into the CDMA mode. Such a communication cutoff causes a great inconvenience for the multi-mode user.
In order to avoid the communication cut of a first method eliminates all WCDMA shadow areas by installing additional WCDMA base stations, at a high cost and over a long period of time. A second method performs a traffic handover using a conventional CDMA network, in which, when the user enters the WCDMA shadow area while communicating, a terminal receives a signal from a base station and switches without cutoff, to the CDMA mode. This method is simple and in expensive in theory, but since the terminal needs to perform handover between mutually different modems, it gets complex and expensive in its hardware and software implementation.
FIG. 1 illustrates a state in which the handover from the WCDMA mode to the CDMA mode occurs.
Reference numeral 300 denotes a dual-mode mobile communication terminal supporting not only a WCDMA mode but also a CDMA mode. Reference numeral 500 denotes a WCDMA base station in service, and reference numeral 700 denotes an adjacent CDMA base station.
The mobile communication terminal 300 is mobile and therefore, can change position. Accordingly, a service providing cell can be changed. In an example illustrating such the state, the mobile communication terminal 300 is positioned at a boundary cell between a WCDMA network and a CDMA network.
FIG. 2 illustrates a construction of the dual mode mobile communication terminal supporting the WCDMA mode and the CDMA mode.
An antenna 211 receives or transmits a WCDMA signal or a CDMA signal. A CDMA radio frequency (RF) unit 212 transmits or receives the CDMA signal through the antenna 211, and processes the transmission and reception CDMA signal. A CDMA processing module 213 transmits or receives the signal from the CDMA RE unit 212, and processes the transmission or reception signals. The CDMA processing module 213 can be a modem chip or a digital signal processor (DSP) chip. A duplexer 214 receives the WCDMA signal and the CDMA signal through the antenna 211, and separates the received WCDMA and CDMA signals. A WCDMA RF unit 215 transmits or receives the WCDMA signal through the antenna 211, and processes the transmission or reception WCDMA signal. A WCDMA processing module 216 transmits or receives the WCDMA signal from the WCDMA RF unit 215, and processes the transmission or reception signals. Like the CDMA processing module 213, the WCDMA processing module 216 can be a modem chip or a DSP chip.
An application processor 218 is a multimedia chip for processing reception and transmission data.
The WCDMA processing module 216 and the application processor 218 are connected through a hardware inter-chip path, and the COMA processing module 213 and the application processor 218 are also connected through a hardware inter-chip path. In the example shown, a communication port of the application processor 218 is connected with the CDMA processing module 213 or the WCDMA processing module 216 through a first switch 217. The application processor 218 and the respective processing modules correspond with each other on a point-to-point basis through the communication port, and the processing module not used is powered off for the while to prevent power consumption.
A universal subscriber identity module (USIM) 219 has user information. A second switch 220 is to connect the USIM 219 with the CDMA processing module 213 or the WCDMA processing module 216 through a universal asynchronous receiver/transmitter (UART) interface.
The above structure is a case where a dual band dual mode (DBDM) terminal is embodied with a single antenna. When even the DBDM terminal is located at a service area providing only the WCDMA service, the CDMA RF unit 212 and the CDMA processing module 213 are temporarily in off states, and only the other elements 211, 214 to 219 are in on states and therefore, the DBDM terminal is operated like the WCDMA terminal. On the contrary, when the DBDM terminal is located at a service area providing only the COMA service, only the WCDMA RF unit 215 and the WCDMA processing module 216 are temporarily in off states, and only the other elements 211 to 214, and 217 to 219 are in on states and therefore, the DBDM terminal is operated like the CDMA terminal.
FIG. 3 is a ladder diagram illustrating the traffic handover between the WCDMA mode and the CDMA mode.
In order to perform the traffic handover from the WCDMA mode to the CDMA mode, two processing modules need to be concurrently operated to transmit a control signal necessary for the handover. Therefore, if the terminal 300 enters the WCDMA shadow area, the WCDMA base station 500 communicates with the CDMA base station 700, and receives information required for the terminal 300 to perform the handover from the WCDMA mode to the CDMA mode. In other words, in Step 3a, the WCDMA base station 500 commands the WCDMA processing module 216 to perform, the handover, and informs the WCDMA processing module 216 of the information for the handover. At this time, the CDMA processing module 213 is powered on and, in Step 3b, a channel is assigned from the WCDMA processing module 216 to the CDMA processing module 213 through an inter-chip communication.
The CDMA processing module 213 of the terminal 300 reads international mobile subscriber identity (IMSI) information from the USIM 219 and while in an idle state. In Step 3c, the CDMA processing module 213 is switched to be in a communication state, using received channel information, and in Step 3d, transmits a handover completion message to the CDMA base station 500 and informs the COMA base station 500 of the execution of the handover.
In Step 3e, the COMA processing module 213 receives a response message (A ck) from the CDMA base station 700 and, if so, in Step 3f, the CDMA processing module 213 completes the handover and informs the WCDMA processing module 216 of the handover completion through the inter-chip communication.
In Step 3g, upon receipt of an intra-chip command of “handover completion” from the CDMA processing module 213, the WCDMA processing module 216 finishes a handover relating operation and is powered off.
FIG. 4 is a ladder diagram illustrating the inter-chip communication for a conventional handover control.
If the CDMA processing module 213 is powered on at a boundary area, it informs the WCDMA processing module 216 of a power on state of the CDMA processing module 213 through an Inform message (Step 4a). The WCDMA processing module 216 transmits an acknowledge (Ack) message in response to the Inform message to the CDMA processing module 213 (Step 4b). If the WCDMA processing module 216 receives the handover command from the base station (Step 4c), the CDMA processing module 213 is powered on and reads the EVISI information from the USIM 219 (Step 4d). After that, if the WCDMA processing module 216 transmits a message of performing the handover to the CDMA processing module 213 (Step 4e), the CDMA processing module 213 informs the WCDMA processing module 216 of a handover result (Step 4f).
After that, connection of the USIM 219 is again made, and the WCDMA processing module 216 is powered off.
As described above, the multi-mode terminal supporting the WCDMA and CDMA modes uses the separate modem chip. In the WCDMA mode, the USIM 219 having the user information is connected with the WCDMA processing module 216. The WCDMA processing module 216 receiving the handover command and the handover information transmits CDMA channel information to the CDMA processing module 213, and connects the USIM 219 to the CDMA processing module 213 through the switch. However, since the CDMA processing module 213 is powered on and then, is first connected with the USIM 219, it transitions to the idle state after reading the IMSI information from the USIM 219. The idle state transitions to the communication state using the channel information. However, since the handover is delayed for the time needed to receive the IMSI information from the USIM 219 through the UART, the handover can fail due to an excess of a handover allowable time of a CDMA standard depending on a network condition.